How long does it usually take for a sinus infection to clear. And how long should you use Flonase spray?

Variable. Most patients gain some relief within 48 hrs if treatment is effective.Actual return to normal may take a week or two. Flonase is not specifically for a sinus infection.It can help if nasal allergies are part of the problem.
It can take. A couple weeks to 3 weeks for a serious infection to clear. Flonase is not effective for acute infections because it takes daily use for 3 weeks to see benefit . It is usually used in chronic infection . In these cases it might be recommended for 6 months or so.
Several days. For your age group 3-5 days to get ahead of it. I use Flonase or similar almost every day as a preventative medication. Systemic effects are relatively minor. Add a vaporizer at the bedside with just plain water. For only a few days you may try adding something like afrin (oxymetazoline) but if you use it frequently you can be addicted to it.